1. Field of the Invention
This invention relates generally to the field of geophysical exploration. More particularly it is concerned with a method and apparatus for determining the permeability of an earth formation by analyzing the propagation of acoustic energy through the formation.
2. Description of the Prior Art
Permeability is a very important reservoir property which cannot be measured directly with presently available logging tools, but rather is measured in the laboratory from core samples or estimated from other well logging information. Conventionally, prior art sonic logs determine properties such as porosity and irreducible water saturation and then calculate permeability using empirical relations obtained from core data. However, the assumed empirical relation may not be applicable in many areas and often the formation properties actually measured cannot be determined with sufficient accuracy.
An acoustic signal transmitted from a logging tool must pass through varying amounts of drilling mud and varying thicknesses of mudcake in order to traverse a length of formation separating the transmitter and receiver of the tool. These factors, together with bore hole irregularities, cause deviations between the true and apparent values of transit time, velocity and degree of attenuation experienced by such an acoustic signal due to the formation itself. Consequently, prior art permeability calculations dependent upon these values are either distorted or require additional steps to correct for such errors.